Reagent test strips are widely used in clinical chemistry. A reagent test strip usually has one or more test areas (pads), and each pad is capable of undergoing a color change in response to contact with an analyte in a liquid specimen. The liquid specimen is reacted with a pad on the reagent strip in order to ascertain the presence of one or more analytes, i.e., constituents or properties of interest, in the liquid specimen. The presence and concentrations of these analytes in the specimen are indicated by a color change in the pads of the test strip when reacted with the analyte. Diffuse light reflected off of the reacted reagent test strip is analyzed to determine the amount of color change. Usually, this analysis involves a color comparison between the reacted test pad and a color standard or scale. In this way, reagent test strips assist medical personnel in diagnosing the existence of diseases and other health problems.
An example of a reagent test strip suitable for use with the present invention is the Glucometer Encore.RTM.--Blood Glucose Test Strips sold by Bayer Corporation, Diagnostics Division, of Elkhart, Ind. 46515.
Reflected light comparisons made with the naked eye can lead to imprecise measurement. Today, reagent strip reading instruments exist that employ reflectance photometry for reading test strip changes. Some reagent strip reading instruments have readheads that contain light emitting diodes (LEDs) for illuminating reagent pads. Some of the light from the LED is reflected off of each pad while some is absorbed in such a way to indicate the color change of the pad due to its reaction with the substance of interest, such as glucose. The diffuse reflected light, i.e., the color-changed light, is detected by a sensor which converts the light into electronic signals for processing.
It has been found that present light emitting diodes (LEDs) are not ideal for use in readheads because the beam of light they produce is not very well collimated. A significant percentage of the diffused light produced by present LEDs tends to become stray light that must be filtered out.
Some prior art inventions have tried to address the problem of stray light being emitted from the LED. One approach has been to encapsulate the sides of the LED with a light absorbing material. An example of a device with such encapsulation is U.S. Pat. No. 5,122,943 by Pugh. This approach results in an LED that absorbs a portion of the light it generates in the encapsulation material.
It would be desirable to have an LED adapted for use in a readhead such that less stray light is produced that requires filtering. Moreover, it would be even more desirable to collimate more of the light that would otherwise become stray light in order to increase the signal and efficiency of the readhead.
However, even when light is fairly well collimated the problem of specular reflection effectively raises the level of "noise" in the light signal received by the sensor. Specular reflection of light is analogous to light bouncing off of a mirror wherein the overall color of the reflected light is not significantly changed. Thus, specular reflection works against sensing a color change of a pad on the reagent strip. It would be desirable to decrease the specular reflection of light received by the light sensor in order to provide a better signal-to-noise ratio.
Because stray light makes sensing the color change of a pad more difficult and less accurate, various optical baffles have been employed to filter some of the stray light. For example, a spiral threaded aperture has been used to reduce stray light. Only light coming from a narrow field of view can travel through the threaded aperture to the sensor, thus stray light is reduced. However, threaded apertures can be costly to form because they require extra manufacturing steps. One way the threaded aperture is formed is by embedding a screw-like element into the plastic as it is being molded. When the plastic has cooled the screw-like element is unscrewed in order to leave a corresponding threaded aperture. Another threaded aperture drawback is that threaded apertures tend to have smaller diameters which reduce the total amount of light received by the sensor, which in turn impacts the sensor's accuracy. Thus, it would be desirable to have an optical baffle that reduces stray light, but is less expensive and easier to fabricate. Furthermore, it is desirable to have an optical baffle that increases the amount of desirable light received by the sensor.